what is capillarization? why is it essential in sports?

we all need blood to be able to live and do our day to day activities but not for the sake of itself, but the oxygen and nutrients it carries. The circulatory system(also called cardiovascular system) is basically a network of tubes that simply shuttles blood around the body. Capillarisation takes place at the alveoli in the lungs and at the skeletal muscle.
This network of blood vessels(arteries, capillaries, and veins) reaches each and every cell of your body. Blood travels in a loop. it gets pumped from heart into arteries and then capillaries(which are small blood arteries/veins), do their interaction with cells(unload oxygen and nutrients and load waste and carbon dioxide) and then enter veins and come back to heart again. this circulatory system includes not just the heart but also 60,000 miles of blood vessels.

Capillaries are important in the delivery of hormones and substrates such as O2, insulin, and glucose to resting and active muscle.

although there are many factors that affect the circulatory system like genetics, lifestyle, diet and so on, exercise is a big factor as well. Those who exercise regularly tend to have greater blood volume.

what is capillarization?

capillarization is The genesis and development of new capillaries inside body and is believed to increased by aerobic exercise. although some researchers believe anaerobic training may promote this process as well, it seems clear that the more oxygen intake you have while training, the more capillarization you have.

capillarization vs mitochondria volume

although beneficial for our body, capillarization is a heavy load of work and is pretty much demanding on the body. also when people first start training, their main problem is not lack of oxygenated and nutrient dense blood, but the ability to use them.
this is why when we come under tension of aerobic exercise, our body first priorities increase in its mitochondria volume since it is not costly from energy perspective(mitochondria are a part of all cells inside our body and they work as tiny motors and supply energy for our cells by taking in glucose and producing adenosine triphosphate or ATP).
our physical capacity to exert effort is an issue of supply and demand on the cellular level. As long as the energy required by a muscle is less than or equal to the muscular ability to produce it, physical effort can be maintained. But when production falls short of demand, fatigue and ultimately failure occur.

aerobic exercise can boost mitochondrial volume by up to 40 percent.

as this process goes on, the body encounters another imbalance of supply and demand. this time our body has the ability to use all oxygenated and nutrient dense blood(because mitochondria has become more capable), but the amount of blood is low. at this point there is no other way around it and we have to go through the taxing process of building new capillaries. this process becomes a loop from this point on, meaning again your cells receive more blood so under more tension, they need to produce more energy so they increase the mitochondria volume again and so on.
one of the main reasons we should focus on capillarization and mitochondria volume, no matter if your focus is aerobic style of training or not, is health and well-being overall. the more blood vessels you have, the easier it is going to be to remove waste from body and keep energy levels up.
this two adaptations also help with anaerobic training massively. since by improving mitochondria volume you produce more energy and capillarization helps you receive and maintain more blood. although for exercises like sprinting, powerlifting or any other type of training that mostly incorporates fast twitch muscle fibres you do not have that much blood flow while in concentric phase, you still have some blood circulation to muscles especially in eccentric phase, also you can keep more blood inside muscle if you have more capillaries.

the other health benefit of capillarization is faster healing process after training or injuries(especially delayed on-set muscle soreness or DOMS).